Abstract
In Arabidopsis, the dehydration-responsive expression of rd29B gene is mainly mediated by abscisic acid (ABA). Promoter analysis indicated that two ABA Responsive Elements (ABREs) function as cis-acting elements, and cDNAs encoding bZIP-type ABRE binding transcription factors, AREB1 and AREB2 were isolated.
AREB1 and AREB2 require ABA for their activation as shown by their low transactivation abilities in protoplasts obtained from the aba2 mutant, suggesting that the AREB1 and AREB2 function as transcriptional activators in ABA-inducible rd29B expression, and ABA-dependent posttranscriptional modification is necessary for maximum activation. In-gel kinase assay revealed that specific ABA-activated protein kinase phosphorylates the conserved regions in the AREBs. Amino-acid substitution (Ser/Thr to Ala) of the putative target sites for CDPK in the conserved region resulted in suppression of both ABA-dependent phosphorylation and transactivation. Substitution of the Ser/Thr to Asp causes the high transactivation activity without ABA. These results suggest that ABA-dependent phosphorylation regulates the AREB transactivation activities.
